1. Field of the Invention
This invention relates to a fuel cell stack, and more particularly relates to a technology which can prevent a degradation in the power generation performance of fuel cell units disposed at both ends of the fuel cell stack.
2. Background Art
A solid polymer electrolyte fuel cell unit, as an example of a fuel cell, comprises: a solid polymer electrolyte membrane, i.e., a cation-exchange membrane; an anode electrode and a cathode electrode which together hold the solid polymer electrolyte membrane therebetween; and a pair of separators which hold the anode and cathode electrodes.
A plurality of solid polymer electrolyte fuel cell units are stacked together to form a fuel cell stack for actual usage.
In such a fuel cell stack, hydrogen gas, as an example of a fuel gas supplied to the anode electrode, is ionized on a catalytic electrode, and moves to the cathode electrode through the solid polymer electrolyte membrane which is moderately moistened. The electrons produced during this process are sent to an exterior circuit, and used as DC energy. At the cathode electrode provided with oxygen gas or air, as an example of an oxidizing gas, water is produced through the reaction of the hydrogen ions, the electrons, and oxygen.
When the fuel cell stack is used in a vehicle, specifically in a passenger car, it is typically disposed under the passenger""s compartment; therefore, the height of the fuel cell stack is tightly restricted.
As a measure to minimize the height of a fuel cell stack, a fuel cell stack is known which has an internal manifold structure in which a plurality of fuel cell units are stacked together in the horizontal direction, and each of the separators is provided with communication ports to form supply ports for a fuel gas, an oxidizing gas, and the like when being assembled (for example, Japanese Unexamined Patent Application, First Publication No. Hei 8-171926).
An example of a fuel cell stack which has this structure will be explained below with reference to FIG. 12. In FIG. 12, reference symbol 1 indicates a fuel cell stack. The fuel cell stack 1 is formed with a plurality of fuel cell units 2 stacked together in the horizontal direction. Each of the fuel cell units 2 comprises: a solid polymer electrolyte membrane; an anode electrode and a cathode electrode which together hold the solid polymer electrolyte membrane therebetween; and a pair of separators which hold the anode and cathode electrodes. Communication ports (not shown) allowing a fuel gas, an oxidizing gas, and a cooling fluid to flow therein are formed in each of the anode electrode and the cathode electrode as through holes; thus, internal manifolds are formed when being assembled.
Each of the fuel cell units 2 is fastened to the others by stud bolts 4.
A fastening structure 5 including coned disc springs or the like is provided at one end of the fuel cell stack 1, in the direction of stacking, and another fastening structure 6 including washers or the like is provided at the other end thereof By means of these fastening structures, the required fastening force is applied to each of the fuel cell units 2 each of which forms a power generation part.
Fuel cell units 2a and 2b located at the ends of the fuel cell stack 1, in the direction of stacking, are provided with terminal plates 7 made of copper which contact their outside surfaces. Each of the fastening structures 5 and 6 is disposed outside each of the terminal plates 7 with an insulation plate 8 therebetween.
A terminal element 9 for outputting electric power extends from the upper portion of each of the terminal plates 7, is bent at a point, and further extends toward the end of the fuel cell stack 1.
Although the fuel cell stack 1 formed as described above is superior in that its height can be minimized as compared to the case in which external manifolds are provided, the space around the fuel cell stack 1 is limited because the terminal elements 9 vertically extend from the fuel cell stack 1.
In addition, because the terminal elements 9 are located near the periphery of the terminal plates 7, the electric resistance tends to be greater as compared to the case in which the terminal elements 9 are located near the centerline of the terminal plates 7.
In order to overcome the above problem, i.e., to minimize the space occupied by a fuel cell stack in both the direction of stacking and the height while maintaining power efficiency, a configuration, hereinafter referred to as modified structure, in which the terminal elements extend in the direction of stacking may be adopted as an example.
However, the modified structure contains a problem, as will be further explained below, due to significant heat dissipation from the terminal plates 7 contacting the fuel cell units 2a and 2b, specifically, significant heat dissipation at the locations from which the terminal elements extend.
FIG. 8 is a graph schematically showing an example of the relationship between the saturated water vapor pressure and the partial pressure of water vapor in the passage for the oxidizing gas, in which the dot-dash line indicates the partial pressure of water vapor, the broken line indicates the saturated water vapor pressure when the terminal plate 7 exhibits a uniform temperature distribution over its surface, and the solid line indicates the saturated water vapor pressure when the terminal element 9 is disposed on the terminal plate 7 so as to be closer to the exhaust end of the passage for the oxidizing gas than to the centerline of the terminal plate 7, which corresponds to the modified structure mentioned above.
FIG. 8 shows that water vapor may more likely be condensed at a location near the exhaust end of the passage for the oxidizing gas than other locations because the partial pressure of water vapor gradually increases toward the saturated water vapor pressure curve as the exhaust end along the passage for the oxidizing gas is approached.
As shown in FIG. 8, in the modified structure, water vapor may more likely be condensed at the location corresponding to the terminal element 9 because the saturated water vapor pressure, which is determined merely depending on temperature, locally decreases around the location corresponding to the terminal element 9.
When the saturated water vapor pressure becomes lower than the partial pressure of water vapor, water vapor condenses, and the condensed water covers the electrode""s reaction surface as a water film. The water film inhibits the electrode""s reaction surface from being supplied with sufficient oxidizing gas, which results in degrading of the power generation performance in the fuel cell units 2a and 2b. 
On the other hand, the above problem due to the condensed water may be prevented by disposing the terminal element 9 at a location far from the exhaust end of the passage for the oxidizing gas; however, as shown in FIG. 9, the electric resistance is increased as the terminal element 9 is disposed farther from the centerline of the terminal plate 7.
Based on the above problems, an object of the present invention is to provide a fuel cell stack in which condensation of water vapor due to a local temperature drop in the end fuel cell units is prevented and the power generation performance of these fuel cell units is effectively maintained while preventing an increase in electric resistance.
In order to achieve the above object, the present invention provides the following.
A fuel stack (e.g., a fuel cell stack 11 in the embodiments) according to a first aspect of the present invention comprises: a plurality of fuel cell units (e.g., fuel cell units 15) stacked together in a direction of stacking each of which comprises an electrolyte element (e.g., a solid polymer electrolyte membrane 12), a pair of electrodes (e.g., an anode electrode 13 and a cathode electrode 14) holding the electrolyte element therebetween, and a pair of separators (e.g., separators 16 and 17) holding the electrodes therebetween; a pair of terminal plates (e.g., terminal plates 21) each of which is disposed outside an end fuel cell unit (e.g., end fuel cell units 15a and 15b) located at an end of the stacked fuel cell units; and a terminal element (e.g., a terminal element 36) extending outwardly in the direction of stacking from the outside surface of at least one of the terminal plates; wherein a location from which the terminal element extends is located within an area on the terminal plate, corresponding to an area in a passage for an oxidizing gas (e.g., a passage 19 for an oxidizing gas) formed in the end fuel cell unit disposed adjacent to the terminal element where the partial pressure of water vapor is lower than the saturated water vapor pressure.
In the fuel cell stack which has the above structure, condensation of water vapor due to a local reduction in the saturated water vapor pressure at the location corresponding to the terminal element may be prevented.
According to a second aspect of the present invention, in the fuel cell stack according to the first aspect, the location from which the terminal element extends is set within the area on the terminal plate so that the total electrical resistance of the terminal element and the terminal plate is minimized.
In the fuel cell stack which has the above structure, condensation of water vapor due to a local reduction in the saturated water vapor pressure at the location corresponding to the terminal element may be prevented, and an increase in electric resistance due to disposition of the terminal element away from the centerline of the terminal plate may also be prevented.
A fuel stack (e.g., the fuel cell stack 11 in the embodiments) according to a third aspect of the present invention comprises: a plurality of fuel cell units (e.g., the fuel cell units 15) stacked together in a direction of stacking each of which comprises an electrolyte element (e.g., the solid polymer electrolyte membrane 12), a pair of electrodes (e.g., the anode electrode 13 and the cathode electrode 14) holding the electrolyte element therebetween, and a pair of rectangular separators (e.g., the separators 16 and 17) holding the electrodes therebetween each of which includes an oxidizing gas exhaust communication port located adjacent to the short side of the rectangular separator; a pair of rectangular terminal plates (e.g., the terminal plates 21) each of which is disposed outside an end fuel cell unit (e.g., the end fuel cell units 15a and 15b) located at an end of the stacked fuel cells; and a terminal element (e.g., the terminal element 36) extending outwardly in the direction of stacking from the outside surface of at least one of the rectangular terminal plates; wherein a location from which the terminal element extends is located farther from the oxidizing gas exhaust communication port than is the centerline of the long side of the rectangular terminal plate.
In the fuel cell stack which has the above structure, because the terminal element is located at a location where the difference between the saturated water vapor pressure and the partial pressure of water vapor is large, condensation of water vapor due to a local reduction in the saturated water vapor pressure at the location corresponding to the terminal element may be prevented.